An increase in a power density or a simplification of a cooling system is considered to further downsize an inverter. It is therefore expected that the inverter needs to be kept constantly operated in a high-temperature environment of not less than 100° C. and not more than 150° C. in the future. A silicon carbide semiconductor device, which is a semiconductor device using a silicon carbide (SiC) layer as a semiconductor layer is appropriate for a high-temperature operation and is expected to satisfy the above demand.
The inverter normally has a switching element such as a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or an IGBT (Insulated Gate Bipolar Transistor), for example. The inverter increases in temperature from a room temperature in accordance with the operation of the switching element. Masahiko Niwayama and the other three describes in “SiC Power Device and Loss-Reduction Operation”, Panasonic Technical Journal, April 2011, Vol. 57, No: 1, pp. 9-14 (non-patent document 1) that a temperature dependence of an on-resistance of a SiC-MOSFET is rendered negative at a temperature equal to or lower than the room temperature and rendered positive at a temperature equal to or higher than the room temperature. In accordance with the above journal, the temperature dependence occurs by a reduction in a channel resistance of MOS and an increase in a drift resistance associated with a temperature increase.
In the SiC-MOSFET, the channel resistance in the on-resistance particularly causes a problem at present. It is considered that a reason why the channel resistance particularly increases at a time of using SiC instead of Si is that an interface state density is high at an interface between an oxide film and a SiC layer in a MOSFET structure, so that a channel mobility is extremely small compared with an electron mobility in a bulk. Accordingly, lowering of the interface state is regarded as important as described in Japanese Patent Application Laid-Open No. 2009-224797 (Patent Document 1), for example.